In geology, an aulacogen or aulocogen is a failed arm of a triple junction of a plate tectonics rift system. A triple junction beneath a continental plate initiates a three way breakup of the continental plate. As the continental break-up develops one of the three spreading ridges typically fails or stops spreading. The resulting failed rift is called an aulacogen and becomes a filled graben system within the continental lithosphere.
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Origin of term
The term "aulacogen" is derived from the Greek aulax (furrow) and was suggested by the Soviet geologist Nikolay Shatsky in 1946.
Characteristics
The crust in an aulacogen region remains weakened by previous rifting activity and thus seismic activity and, occasionally, volcanic activity may re-occur subsequently from time to time. As aulacogens remain places of weakness, given the appropriate conditions, they can reactivate into active rift valleys again, as had happened to the Ottawa-Bonnechere Graben in Ontario and Quebec, Canada, an ancient aulacogen that reactivated during the breakup of Pangaea. Abandoned rift basins that have been uplifted and exposed onshore, like the Lusitanian Basin, are important analogues of deep-sea basins located on conjugated margins of ancient rift axes.